This invention relates to an antenna assembly for an induction logging tool.
The invention has been developed primarily, though not exclusively, with a view to provide an improved antenna assembly for an induction logging tool of the general type disclosed in more detail in U.S. Pat. No. 6,100,696 and PCT/GB00/02510. However, the improved antenna assembly of the invention is applicable generally to induction logging tools.
In designing an antenna assembly for an induction logging tool, consideration has to be given to the external environmental conditions which prevail during a typical drilling situation e.g. pressures of up to 20,000 p.s.i., temperatures ranging from −35° C. to +175° C., and shock and vibration to 1000 g. Despite the environmental factors it is important to maintain electrical stability, and in particular self inductance of the tool in order to achieve accurate logging.
Conventional wisdom suggests that it is not possible to employ magnetic material in an induction logging tool, in a drilling environment, and while maintaining electrical stability of the antenna assembly. Typically, antenna instability in an induction logging instrument results in diminished accuracy of the measurement.
In addition, typically antenna instability in an induction logging instrument results in diminished accuracy of the logging measurement. Furthermore, MPP (molypermalloy)- used in logging tools, is a brittle material and is at risk of being damaged in a drilling environment.
The use of magnetic materials in a coil also effectively permits a reduction in antenna dimensions, more particularly the area enclosed by the wound coil. In this case, an effective permeability of between 5–6 (produced by the magnetic core) allows a reduction in coil diameter of around 2.5, for same antenna sensitivity.